What about nuclear fusion?

We say that we will put the sun into a box. The idea is pretty. The problem is, we don’t know how to make the box.

S´ebastien Balibar, Director of Research, CNRS

Fusion power is speculative and experimental. I think it is reckless to
assume that the fusion problem will be cracked, but I’m happy to estimate
how much power fusion could deliver, if the problem is cracked.

The two fusion reactions that are considered the most promising are:

the DT reaction, which fuses deuterium with tritium, making helium; and

the DD reaction, which fuses deuterium with deuterium.

Deuterium, a naturally occurring heavy isotope of hydrogen, can be ob-
tained from seawater; tritium, a heavier isotope of hydrogen, isn’t found
in large quantities naturally (because it has a half-life of only 12 years) but
it can be manufactured from lithium.

ITER is an international project to figure out how to make a steadily-
working fusion reactor. The ITER prototype will use the DT reaction. DT
is preferred over DD, because the DT reaction yields more energy and be-
cause it requires a temperature of “only” 100 million °C to get it going,
whereas the DD reaction requires 300 million °C. (The maximum temper-
ature in the sun is 15 million °C.)

Let’s fantasize, and assume that the ITER project is successful. What
sustainable power could fusion then deliver? Power stations using the DT
reaction, fuelled by lithium, will run out of juice when the lithium runs
out. Before that time, hopefully the second installment of the fantasy will
have arrived: fusion reactors using deuterium alone.

I’ll call these two fantasy energy sources “lithium fusion” and “deu-
terium fusion,” naming them after the principal fuel we’d worry about
in each case. Let’s now estimate how much energy each of these sources
could deliver.

Lithium fusion

World lithium reserves are estimated to be 9.5 million tons in ore deposits.
If all these reserves were devoted to fusion over 1000 years, the power
delivered would be 10 kWh/d per person.

There’s another source for lithium: seawater, where lithium has a con-
centration of 0.17 ppm. To produce lithium at a rate of 100 million kg
per year from seawater is estimated to have an energy requirement of
2.5 kWh(e) per gram of lithium. If the fusion reactors give back 2300 kWh(e)
per gram of lithium, the power thus delivered would be 105 kWh/d per
person (assuming 6 billion people). At this rate, the lithium in the oceans
would last more than a million years.

Figure 24.15. The inside of an experimental fusion reactor. Split image showing the JET vacuum vessel with a superimposed image of a JET plasma, taken with an ordinary TV camera. Photo: EFDA-JET.

Figure 24.16. Lithium-based fusion, if used fairly and “sustainably,” could match our current levels of consumption. Mined lithium would deliver 10 kWh/d per person for 1000 years; lithium extracted from seawater could deliver 105 kWh/d per person for over a million years.